trivial: just avoid to hardcode the build directory that can potentially be modified in the pom

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/fileupload/trunk@1461327 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/src/main/assembly/src.xml b/src/main/assembly/src.xml
index 901ffe7..d5152cb 100644
--- a/src/main/assembly/src.xml
+++ b/src/main/assembly/src.xml
@@ -34,7 +34,7 @@
         <exclude>**/doap_*.rdf</exclude>
         <exclude>**/bin/</exclude>
         <exclude>**/.externalToolBuilders/</exclude>
-        <exclude>**/target/</exclude>
+        <exclude>**/${project.build.directory}/</exclude>
         <exclude>**/download*.cgi</exclude>
       </excludes>
     </fileSet>